1. Field of the Invention
The present invention relates to focus control of an image-taking apparatus.
2. Description of the Related Art
A hybrid auto-focus (AF) control method is one of AF control methods for use in image-taking apparatuses such as a video camera. As disclosed in Japanese Patent Application Laid-Open No. 2002-258147, the hybrid AF control method is a combination of a so-called TV-AF method in which a focusing lens is controlled to maintain the maximum level of an AF evaluation value signal provided by extracting a high-frequency component (a contrast component) from an output signal of an image pickup element with a control method in which the drive amount of a focusing lens is calculated on the basis of a signal from a sensor which measures the distance to an object or a defocus amount.
The TV-AF method allows high focusing accuracy but it is necessary to take a long time to achieve focusing to search for the lens position at which the AF evaluation value signal is at the maximum. In the hybrid AF control method, the signal of the separately provided sensor is used for reference to seek a reduction in the time to achieve focusing.
Japanese Patent Application Laid-Open No. 2002-258147 has also proposed, as an exemplary structure of the hybrid AF, a combination of the TV-AF method with a TTL (Through The Lens) method in which light incident on an image-taking optical system is split and received by sensors to measure a defocus amount based on signals which represent the received light. In addition, Japanese Patent Application Laid-Open No. 2002-258147 has proposed a combination of the TV-AF method with an external metering method in which a distance sensor is provided separately from an image-taking optical system.
In the external metering method, a so-called parallax problem is known due to a mismatch between the optical axis of the image-taking optical system and the measuring axis of the distance sensor. For example, when a zoom lens is set on a telephoto side, the field angle for taking images is reduced, and the distance sensor may detect a substance out of the field angle as a target object.
For a camera having the maximum zoom magnification which is relatively low, however, the field angle for image-taking is not very small and only slight effect is caused. Japanese Patent No. 3381233 has proposed a means for overcoming the parallax problem by using a sensor which emits a radiation beam and receives the reflected beam from an object as a distance sensor to determine whether or not the object is placed on an optical axis from the relationship between the emergence angle and the incident angle of the reflected beam. While this means is disadvantageous in cost as compared with a distance sensor used in a conventional compact camera or the like, a hybrid AF control method can be realized which does not perform erroneous detection of an object due to parallax in a camera having a high level of the maximum zoom magnification.
Recent digital cameras and video cameras have an electronic zoom function in which electric processing is used to enlarge a part of an object image taken by an image pickup element such as a CCD sensor and a CMOS sensor for display and record. The electronic zoom function achieves zooming through electric processing beyond the optical zoom magnification of an image-taking optical system (approximately 10 to 20 times in a video camera) to allow pseudo zooming up to a zoom magnification of several tens to 200 times or more.
When such an electronic function is used to enlarge an area of video through electric processing, and the TV-AF control method or the hybrid AF control method involving the TV-AF control method is applied to the enlarged video area, the following problem arises.
Specifically, since only part of the video signal of an object is used in an electronic zoom state, a smaller area of the video is used for calculating the AF evaluation value signal than in a non-electronic zoom state. With the small video area, a slight movement of an object or a small shake of a hand which holds a camera largely changes the video of the object in the video area.
As described above, focusing control is performed in the TV-AF control method by searching for the lens position at which the AF evaluation value signal is at the maximum. The AF evaluation value signal, however, varies with a change in video of an object, so that a large change in the video may lead to erroneous operation of the AF control. For this reason, in the hybrid AF control method involving the TV-AF control method, degraded AF performance in the electronic zoom state presents a problem.
Detailed description is hereinafter made for the problem with reference to FIGS. 2A to 2C. FIG. 2A shows an image taking area in the non-electronic zoom state. In the TV-AF, signal processing is performed on the video signal within a predetermined area (an AF frame), for example near the center, of the image taking area to calculate the AF evaluation value signal for use in AF control.
On the other hand, FIG. 2B shows an example of an image taking area in the electronic zoom state, and FIG. 2C shows an example of an area enlarged and displayed in the electronic zoom state. Since the video in the enlarged area shown in FIG. 2C is displayed and recorded, an AF frame is set in the enlarged image taking area shown in FIG. 2C to have the same size as that in FIG. 2A. In this case, as shown in FIG. 2B, the size of the AF frame in the actual image taking area is smaller than that in the non-electronic zoom state. In this manner, the AF frame becomes smaller as the electronic zoom magnification becomes higher, and consequently, erroneous operation of the TV-AF control easily occurs due to a change in video of an object as described above.